a) Field of the Invention
The present invention relates to a directional coupler having both capacitances and mutual inductance (CM type) used as a power detector in a mobile radio communication apparatus such as portable radio telephone and the like for control of the transmission output thereof.
b) Description of the Prior Art
In the mobile radio communication apparatuses such as a portable radio telephone, the transmission power must be controlled to an appropriate value in order to prevent an interference between portable apparatuses, communicate with the base station on an optimum transmission power, prevent an intermodulation distortion from taking place, to minimize the battery power consumption or for the similar purposes.
Because of the portability of such radio apparatus, the antenna impedance the transmitter sees is not stable as the apparatus is handled in various manners when in use, as held in hand, moved to the face of user or held in excessively inclined position. Therefore, the power detector used for control of the transmission output must be adapted to detect a predetermined level of power irrespective of any load variation. Namely, it must have such a directivity that it can detect only the power of an incident wave upon the antenna while ignoring the power of a reflected wave. A variety of power detectors have so far been proposed for the above purpose. They are generally classified into the following groups according to their configurations:
(a) Distributed coupling lines are formed on the printed circuit board in the radio communication unit. PA1 (b) For detection of the transmission power by voltage division, an isolator is provided between the transmitter and antenna. PA1 (c) Capacitors and coils are discretely disposed to build a CM type directional coupler. PA1 (a) Since a portion of the printed circuit board is used for provision of the coupling lines, it takes much time to develop and design such a circuit board. PA1 (b) The isolator itself is expensive. PA1 (c) Since circuit elements such as capacitors, coils, etc. are discretely disposed on a circuit board, it is difficult to assure an appropriate directivity. Also it is extremely difficult to manufacture such a directional coupler. Likewise the compact designing is difficult.
However, such directional couplers are disadvantageous in the following respects: